Signal amplification is widely used in a variety of electronic systems. For example, in wireless communication systems, a low-noise amplifier (LNA) in the receiver amplifies the tiny signal picked up by an antenna. The amplified signal is then filtered, demodulated and further amplified again. The signal is often converted to a digital signal by an analog-to-digital converter. The analog-to-digital converter also typically amplifies the signal in the conversion process. In the transmitter, the power amplifier amplifies the radio frequency signal before the signal is transmitted through the antenna. In systems employing sensors, such as image sensors, microphones, and micro-electromechanical sensors, the signal produced by the sensors is very small, and thus must be amplified before further signal processing. There are numerous other places where signal amplification is required in a system. Conventional amplifier circuits fall into three general categories: a common-emitter amplifier (a common-source amplifier in MOS technologies), a common-based (a common-gate amplifier in MOS technologies), and an emitter-follower amplifier (a source-follower amplifier in MOS technologies). The first two types typically provide a substantial amount of voltage gain. However, the last type provides voltage gain that is close to one or slightly less, and as a result it has been suitable only as a buffer amplifier, but not as a voltage amplifier.
As an example of a simple circuit according to the prior art, FIG. 1 provides an illustration of a representative common-source amplifier circuit 20 which operate in continuous-time. The circuit 20 also includes a MOSFET M1, and a load resistor RL. An analog input voltage vIN (hereafter referred to as “input voltage”) provides an input to the circuit 20, and a voltage vO (hereafter referred to as “output voltage”) is provided as an output of the circuit 20. The input voltage VIN generally includes a DC bias component VIN and a small-signal component vin (hereafter referred to as “input signal”) such that vIN=VIN+vin. Likewise, the output voltage vO generally includes a DC component VO and a small-signal component vo (hereafter referred to as “output signal”) such that vO=VO+vo. The DC components are required to bias the MOSFET in the desired region of operation. The small-signal components are typically the signals of interest. The circuit 20 amplifies the input signal vi such that the output signal vo is given byvo=−gmRLvin where gm is the transconductance of M1. The small-signal voltage gain (hereafter referred to as “voltage gain”), defined as the ratio between the output signal vo and the input signal vi is then
      a    v    =                    v        o                    v        in              =                  -                  g          m                    ⁢                        R          L                .            
Further analysis of the circuit 20 shows that the frequency fh where the magnitude of the voltage gain drops by 3 dB's from the low frequency value (hereafter referred to as “bandwidth”) is given by
      f    h    =      1          2      ⁢      π      ⁢                          ⁢              R        L            ⁢              C        L            where CL is the total capacitance at the output node. A figure-of-merit, the gain-bandwidth product GBW, of an amplifier is defined as the product between the low-frequency gain and the bandwidth. For the circuit 20, it is given by
  GBW  =                                    a          v                ⁢                  f          h                            =                            g          m                          2          ⁢          π          ⁢                                          ⁢                      C            L                              .      